In recent years, aircrafts are becoming larger due to the increasing number of joined parts. In order to machine such a large sized component, a machine tool such as disclosed in Japanese Patent No. 3939095 is often utilized which comprises a spindle supporting side structure erected on a floor surface for movably supporting a spindle and a work supporting side structure for supporting a work also erected on the floor surface in opposition to the spindle supporting side structure and in which the work is machined by relative movement of a tool mounted on the spindle supported by the spindle supporting side structure and the work supported by the work supporting side structure.
The spindle supporting side structure and the work supporting side structure as described above are generally large, and have a cantilever configuration with one end on the floor surface side fixed to the floor surface and with the other opposite, upper end being free. Thus, when a force or cutting load is imposed between the spindle supporting side structure and the work supporting side structure via contact of the tool with the work at the time of machining, deflection may be produced in the spindle supporting side structure as well as in the work supporting side structure. In particular, the higher the position of the action point of the force, the larger is the amount of deflection or deformation of the spindle supporting side structure and the work supporting side structure. Deflection may also be produced by local thermal deformation of the spindle supporting side structure and the work supporting side structure due to temperature variation in the room in which the machine tool is installed. The amount of the deformation is largest at the upper, free end of the structures. Further, deformation of these structures may be produced due to the weight or the movement of the spindle supported by the spindle supporting side structure as well as of the work supported by the work supporting side structure.
A problem arises that, if the relative distance between the upper end of the spindle supporting side structure and the upper end of the work supporting side structure changes in this manner due to deformation of the spindle supporting side structure and the work supporting side structure, error may be produced in the relative positional relation of the spindle and the tool relative to the work, which may cause degradation of machining precision. In particular, when the deflection of the spindle supporting side structure occurs in an opposite direction to the deflection of the work supporting side structure, the change in the relative distance between the upper end of the spindle supporting side structure and the upper end of the work supporting side structure becomes large so that the adverse influence on the machining precision also may become large.